1. Field of the Invention
The present invention relates generally to thermal printing apparatus for printing gray scale images and, more particularly, relates to apparatus for producing gray scale image signals to be fed to a thermal printer to produce a printed copy of a still image, as represented by one frame of a television signal.
2. Description of Prior Art
It is known, according to the prior art, to produce a printed copy of a frame of a television signal based on the gray scale levels of the television frame using an electrically energizable thermal head. The various gray scale levels in the frame are obtained by varying the period of time during which the several elements of the thermal head are energized, the period of energization being proportional to the detected gray scale levels of the elements of the television frame.
For example, and referring to FIG. 1, when producing a printed copy of an image on television screen S, the video signal corresponding to the picture elements of the image on the television screen are scanned or examined sequentially. Such scanning is vertically in the downward direction, as indicated by arrows A and B, in order to sample or measure the gray scale level of each of the picture elements d.sub.1,1 to d.sub.256,1 in the case of line A and d.sub.1,2 to d.sub.256,2 in the case of line B. All of the subsequent remaining vertical lines of the frame are scanned in a similar fashion. To obtain values corresponding to the gray scale levels, the levels corresponding to the various sample points are converted to pulse width modulation (PWM) signals, and are used to drive a line of heat elements, h.sub.1 to h.sub.256, of thermal head unit 1. By pulse width modulating the energization signals based upon the detected gray scale levels of the picture elements, the heat elements of the thermal head will be powered for a corresponding length of time to reproduce the various gray scale levels forming the television image.
Referring now to FIG. 2, a known gray scale signal generating circuit is shown in block diagram form. In FIG. 2, a sample voltage corresponding to a general picture element d.sub.m,n, as shown in FIG. 1, is supplied to terminal 2 of analog-to-digital converter 3. The digital output signals of analog-to-digital converter 3 are stored in units of lines, corresponding to the lines in the television frame S, in random access memory (RAM) 4 at locations in accordance with address signals generated from address counter 5. The stored gray scale level signals are then sequentially read out from random access memory 4 into comparator 6, where they are compared with respective reference gray scale signals from gray scale counter 7. Thus, the output of comparator 6 represents a series of adjusted gray scale levels corresponding to the various scanned points of the image of the television screen S having been compared with reference gray scale signals. The outputs of comparator 6 are latched into positions in addressable latch 8, as determined by the output of address counter 5, which also provides the addresses that are input to random access memory 4. The output signals from addressable latch 8 are then fed to thermal head 1 as parallel signals corresponding to the number of energizable heat elements that make up thermal head 1. The various signals are clocked through the system of FIG. 2 in a synchronous fashion under control of clock signal generators 9 and 10.
In the gray scale signal generator described above in relation to FIG. 2, the picture elements in one vertical line of the television frame that are to be copied are fetched or retrieved as digital signals in accordance with an address signal from address counter 5. The digital signals are then compared in sequence with the reference gray scale level signals produced by gray scale counter 7 in comparator 6. The result of each comparison is a binary output signal, consisting of either "1" or "0", produced in accordance with the comparative intensity of the two signals being individually compared. The binary output signal from comparator 6 is fed to addressable latch 8. Because the picture element data of one vertical line of television screen S must be supplied to comparator 6 in response to each reference gray scale signal, the circuit configuration must be quite complex in order to perform this comparison, and such complexity results in a constraint on the operating speed of the system. This circuit complexity not only adversely affects system operating speed but also decreases overall circuit reliability.